help!!!!!!!

ok my comp lags on everything diablo 2 the internet, myspace, even runescape , the only thing it sems to run without lag is windows live messenger and windows media player and on top of that i have a 512 mb ram and it shows i have 446 mbs of ram i called a comp repar person and he said i have my onboard video card enabled well hes wrong its disabled and still having problums he also said that my firewalls blocking every thing wrong agen its only blocing 1 thing at the moment still lag i have trend micro anti virus it works good so can anyone help me plzzzzzzzzz